1. Technical Field
The present invention relates to the structure of a disk clamp which rotatably and securely holds recording disks that rotate at a high speed in an disk recording apparatus. The disk clamp connects the disks to a spindle motor, which is a rotary drive.
2. Description of the Prior Art
A hard disk drive, which is an disk recording apparatus in which recording media is magnetic recording disks. The magnetic recording disks rotate at a high speed in an recording apparatus used in an information processing apparatus such as a computer. More than one magnetic recording disk (hereafter referred to as a recording disk) rotates at a high speed and writes or reads information data by means of magnetic heads individually disposed for the top and bottom faces of the recording disks.
A recording disk for a hard disk drive is rotated by a spindle motor, which is a rotary drive. A recording disk rotates at a speed as high as a few thousand revolutions per minute. Therefore, a disk clamp that secures each recording disk to the spindle shaft of the spindle motor must have a structure and strength for holding each recording disk.
A hard disk drive of 3.5-inch type, for instance, may not change its dimensions but is demanded to increase the storage capacity by several tens of percent or greater each year, and is demanded to reduce the thickness and the other outer dimensions as much as possible to keep up with the miniaturization of internal units or the like. Accordingly, disk clamp has also been desired to reduce the thickness and the other outer dimensions as much as possible. But a certain physical dimensions in the disk clamp are required to securely hold the recording disks and stand their high-speed revolutions, so that it is difficult to make the current dimensions smaller or slimmer.
On the other hand, the recording disk, like a compact disc for instance, on which recording is performed with magnetically, in that it has the disc shape and a hole at its center. And the recording disk has a large number of magnetic recording tracks which are concentrically formed on its surfaces. To satisfy the recording density which has been increasing year after year, the recording disk is required to have a highly flatness level. The surfaces of a glass substrate can be easily smoothed out. So, the glass substrate become more popular than the conventional aluminum substrate in recent years.
FIG. 10 is a cross-sectional view showing the structure of a conventional disk clamp which holds recording disks using three glass substrates. The disk clamp shown in FIG. 10 holds three recording disks 17(A), 17(B), and 17(C) together from the top and bottom. The member that holds the recording disk 17(C) from the bottom is the stainless steel hub 23, and the member that holds the recording disk 17(A) from the top is the stainless steel top clamp 21. Stainless steel is one of materials that are stable in terms of chemical properties over the whole temperature range in which the disk recording apparatus (hard disk drive) would be used. And the stainless steel has such a coefficient of elasticity that a clamping force required to clamp the recording disks 17(A) to 17(C) while the disk recording apparatus(describe later) is in use. The clamping force can be obtained from the tightening force to tighten the screws 22. The hub 23 is secured to the spindle shaft 18, which is the axis of rotation of the spindle motor 25. The top clamp 21 is secured by tightening the screws 22 into the hub 23. In the spaces among the three recording disks 17(A), 17(B), and 17(C), the ring-shaped spacers 24 formed by a ceramic material are inserted. The thermal expansion coefficient of the spacers 24 is close to that of the glass substrate.
The radius of the cylindrical portion 23a of the hub 23 that goes through the center holes of the recording disks 17(A), 17(B), and 17(C) is smaller than the radius of the perimetric portion 23b which holds the recording disk 17(C) from the bottom. Likewise, the radius at which screwing positions 21a are disposed in the top clamp 21 is smaller than the radius of the perimetric portion 21b which holds the recording disk 17(A) from the top.
The screwing positions 21a and the perimetric portion 21b of the top clamp 21 are integrally formed in a stainless steel member, and the thickness of the connecting portion 21c is L1. The cylindrical portion 23a and the perimetric portion 23b of the hub 23 are also integrally formed in a stainless steel member, and the thickness of the connecting portion 23c is L2.
FIG. 11 illustrates that the tightening force by the screws 22 for securing the recording disks 17 shown in FIG. 10 is transferred through the disk clamp up to the perimetric portion 21b or 23b. The tightening force FC1 of the screws 22 travels through the cylindrical portion 23a of the hub 23 and works to press the cylindrical portion 23a and the screwing position 21a in a direction to close each other, as represented by the arrows in FIG. 11. The tightening force FC1 also travels through the connecting portion 21c of the top clamp 21 up to the perimetric portion 21b, but the magnitude of this force varies with the distance L3, coefficient of elasticity (Young""s modulus), and thickness L1. For instance, the tightening force FC2 in the perimetric portion 21b grows weaker with increase in distance L3, with decrease in coefficient of elasticity, or with decrease in thickness L1. The tightening force traveling through the connecting portion 21c is defined as transfer force Ml. The magnitude of the transfer force M1 of the connecting portion 21c is proportional to the thickness L1 of the connecting portion 21c if the material has uniform properties, like stainless steel. The transfer force M1 traveling in this way is transferred by the perimetric portion 21b to the clamp portion 17a of the recording disk 17(A) and combined with the force transferred by the perimetric portion 23b of the hub 23 (described later), to form the tightening force FC2, which presses the clamp portion 17a from the top.
Likewise, the tightening force FC1 which travels through the cylindrical portion 23a of the hub 23 is transferred through the connecting portion 23c by distance L3 up to the perimetric portion 23b. The transfer force through the connecting portion 23c is defined as M2. The transfer force M2 varies with the distance L3, coefficient of elasticity (Young""s modulus), and thickness L2. And the transfer force M2 is proportional to the thickness L2 of the connecting portion 23c, for instance, if the material has uniform properties. The transfer force M2 traveling in this way is transferred by the perimetric portion 23b to the clamp portion 17c of the recording disk 17(C). And the transfer force M2 is combined with the force from the perimetric portion 21b of the top clamp 21, which was described earlier, to form the tightening force FC2. The FC2 presses the clamp portion 17b from the bottom.
The tightening force FC2, which is the resultant of the force pressing the clamp portion 17a of the recording disk 17(A) from the top and the force pressing the clamp portion 17b of the recording disk 17(C) from the bottom, is transferred through the recording disks 17(A), 17(B), and 17(C) and the spacers 24 among the recording disks 17(A), 17(B), and 17(C) and secures the recording disks. The spacers 24 are inserted to maintain spaces between the recording disks 17(A) and 17(B) and between 17(B) and 17(C) and are made of ceramics of which thermal expansion coefficient is almost the same as that of the recording disks 17.
The hard disk drive is generally used in an environment of room temperature (about 20 to 25 degrees Celsius), and the internal temperature rises to about 50 to 60 degrees Celsius by the heat generated by the rotation of the spindle motor 25 and the recording disks 17, the driving of the voice coil motor, and the like. Room temperature may also decrease to about 0 degree Celsius. Therefore, the hard disk drive would generally be placed in an environment of temperature cycling of about 0 to 60 degrees Celsius, and the recording apparatus would be used in the temperature range. The temperature range becomes the whole temperature range of the recording apparatus. In FIG. 11, the direction of the largest thermal expansion caused by the temperature rise in each recording disk 17 is indicated as TE1. In the temperature cycling environment, thermal expansion and contraction is large in the direction TE1.
3. Problems to be Solved by the Invention
Because the thermal expansion coefficient of stainless steel is approximately is different from the thermal expansion coefficient of glass substrate, the width of expansion or contraction by thermal expansion of the perimetric portion 21b is different from the width of expansion or contraction by thermal expansion of the clamp portion 17a of the recording disk 17 in the temperature cycling environment. As a result, if the temperature rises after the glass substrate is mounted on the disk clamp, the perimetric portion 21b having a large thermal expansion coefficient would shift the point of contact with the clamp portion 17a from the first position of contact toward the radially outer side of the recording disk 17, so that a radially outward stress would be applied to the recording disk 17. Then, if the temperature decreases to room temperature, the perimetric portion 21b would return the point of contact with the clamp portion 17a to the original position of contact, so that a radially inward stress would be applied to the recording disk 17.
However, the top clamp 21 and hub 23 are secured by the screws 22, as described above, and the number of positions secured by the screws 22 are limited, so that when the whole perimetric portion 21b of the top clamp 21 is viewed, the distance L3 between a position of the perimetric portion 21b and its nearest screw 22 depends on the position. Supposing that the position of the screw 22 is a fixed center and that the perimetric portion 21b is a point of action for securing the recording disk the distance between the center and the point of action depends on the position in the perimetric portion 21b. A difference in distance results in a difference in the force applied to the point of action (tightening force), as described above.
Stainless steel used for the disk clamps shown in FIGS. 10 and 11 has so large a coefficient of elasticity (Young""s modulus) and so high an elastic limit (almost equal to the proportional limit) that the whole perimetric portion 21b of the top clamp 21 can be kept relatively uniform even if it is secured by the screws 22. However, the pressing force of the perimetric portion 21b is slightly different, depending on the distance from the screws 22, as described above. The difference causes this problem: After temperature cycling is conducted as described above, the recording disk 17(A) or 17(C) that thermally expanded in the direction TE1 is likely to return to the original position of contact with the perimetric portion 21b or 23b, in positions far from the screws 22, and the recording disk 17(A) or 17(C) that thermally expanded in the direction TE1 is not likely to return to the original position of contact with the perimetric portion 21b or 23b, in positions near the screws 22.
The recording disks 17 of the hard disk drive have recording tracks formed beforehand in the magnetic recording faces and are secured to the spindle shaft by the disk clamp (top clamp 21 and hub 23). Therefore, after temperature cycling is carried out, the recording tracks in the magnetic recording faces of the recording disk 17(A) or 17(C) are likely to return to the original positions of contact with the perimetric portion 21b or 23b in some places and are not in the other places, as described above. That is, the recording tracks that were almost round about the spindle shaft when they were formed beforehand in the magnetic recording faces of the recording disk 17 would become wavy circles affected by differences in distance from the screws 22.
FIG. 12(a) shows a record of fluctuations in data position of a single full track within the width of a single track of the recording tracks on the recording disk (A) after a temperature cycling test is carried out with a disk clamp with six screws 22, and the value of this PES (position error signal) is set as a relative reference value of 100. FIG. 12(b) shows the distribution of data positions within the track width of a single full recording track shown in FIG. 12(a), and the standard deviation is set as a relative reference value of 1.0, and the corresponding value of RRO (repeatable run out) is also set as 1.0.
A single cycle of the temperature cycling test is, for instance, to raise the ambient temperature of a test object from 25 to 60 degrees Celsius, back to 25 degrees Celsius, then down to 0 degree Celsius, and back again to 25 degrees Celsius, for instance, and the test is carried out to cover the whole temperature range in which the recording apparatus would be used as described above.
In FIGS. 12(a) and 12(b), the width of a single track is divided into 256 parts, and the center of the recording track corresponds to the position of 128 on the vertical axis in FIG. 12(a) or the position of 128 on the horizontal axis in FIG. 12(b). When the disks are first secured by the disk clamp, all data positions in a single full recording track formed in the recording disk 17 almost match the position of 128 in the figures described above or remain within a very narrow range around the position, decreasing the values of standard deviation and RRO mentioned above.
As shown in FIG. 12(a), the data positions of a full recording track after temperature cycling have six peaks P1 to P6 of positional deviation. Because the peaks P1 to P6 of positional deviation occur in almost identical positions in the six individual cycles f1 to f6 of 60 degrees, it is apparent that the positional deviations occur depending on the angle. In the example shown in FIG. 12, the six screws 22 are placed at intervals of 60 degrees in the range of 360 degrees, so that it is clear that the peaks PI to P6 of the positional deviation correspond to the positions of the screws 22. Therefore, it is found that the conventional stainless steel disk clamp (top clamp 21 and hub 23) causes the peaks P1 to P6 of positional deviation in data position of a recording track to occur after a temperature cycling test is executed, corresponding to the positions of the screws 22.
If the disk clamp shown in FIG. 12(a) is used, the extension of the data position distribution corresponds to a relative reference value of 100 within the 256 divisions of a single track, as shown in FIG. 12(b). Some data positions in a recording track such as the peaks P1 to P6 are largely deviated from the position of 128, which is the center of the track.
To write or read data in a data position largely deviated from the center of track, the servo and other technologies must be used to move the head. Sudden movements of the head, however, are not desirable because there is a danger of unsuccessful data read or write, even if the servo and other technologies are used. For stable read or write by the head, data positions should be centered in and normally distributed within a narrow range about the center position of 128. In the example shown in FIG. 12(b), the distribution of data positions is spread to the extent very far from the center position of 128, so that it would be hard to read or write data with stability.
The present invention has been provided to ease or solve the problems as described above, with an object of providing such a disk clamp that data positions in recording tracks are resistant to positional deviation even after temperature cycling is conducted and that the distribution of data positions is centered in the vicinity of the center.
To solve the above problem, a disk clamp for information recording disk apparatus of the present invention is a disk clamp which secures recording disks with circumferential recording tracks formed beforehand on disk-shaped glass substrates to the spindle shaft in the information recording disk apparatus, and comprises at least a cylindrical hub secured to the spindle shaft, a top clamp which holds from the top the recording disks concentrically disposed on the hub, and screws which secure the top clamp to the hub; and the top clamp and the hub are made of materials that are stable in terms of chemical properties over the whole temperature range in which the disk recording apparatus would be used and have such a coefficient of elasticity that a clamping force required to clamp the recording disks while the disk recording apparatus is in use can be obtained from the tightening force to secure the screws; and the material of either the top clamp or the hub at least has a thermal expansion coefficient close to that of the glass substrate.
In another aspect of the invented disk clamp is made by titanium as the main material at least either one of the top clamp or the hub.
In another aspect of the invented disk clamp, when the main material of just either the top clamp or the hub is made by titanium, the material of the other is made by stainless steel.
In another aspect of the invented disk clamp, when a plurality of glass substrates are provided, a ring-shaped spacer made of a ceramic material of which thermal expansion coefficient is close that of the glass substrate is inserted between the glass substrates.
In another aspect of the invented disk clamp for information recording disk apparatus of the present invention is a disk clamp which secures recording disks with circumferential recording tracks formed beforehand on disk-shaped glass substrates to the spindle shaft in the information recording disk apparatus, and comprises at least a cylindrical hub secured to the spindle shaft, a top clamp which holds from the top the recording disks concentrically disposed on the hub, screws which secure the top clamp to the hub, and a ring-shaped thermal strain buffer which is inserted into the portion of contact between the hub or top clamp at least and the recording disk; and the thermal strain buffer is made a material that is stable in terms of chemical properties over the whole temperature range in which the disk recording apparatus would be used and has a thermal expansion coefficient close to that of the glass substrate.
In another aspect of the invented disk clamp, the main material of the thermal strain buffer is made by titanium.
In another aspect of the invented disk clamp, the thermal strain buffer is formed by punching a ring out of a thin titanium sheet evenly spread by forging or rolling.
In another aspect of the invented disk clamp, the thickness of the thermal strain buffer is 0.2 mm or smaller.
In another aspect of the invented disk clamp, alpha-type titanium is used for the thermal strain buffer.
FIG. 1 is a top view showing a magnetic recording disk apparatus having the disk clamp of the first embodiment of the present invention.
FIG. 2 is a sectional view of the disk clamp shown in FIG. 1.
FIG. 3 shows the PES values of the disk clamp shown in FIG. 1, obtained by experiments.
FIG. 4 is a sectional view of the disk clamp of a second embodiment of the invention.
FIG. 5 is a sectional view of the disk clamp of a third embodiment of the invention.
FIG. 6 is a sectional view of the disk clamp of a fourth embodiment of the invention.
FIG. 7(a) shows a record of fluctuations in data position in a single full recording track after temperature cycling is conducted with the disk clamp having six screws of this embodiment, and
FIG. 7(b) shows the distribution of data positions in a single full recording track shown in FIG. 7(a) within the width of the track.
FIG. 8 is a sectional view of the disk clamp of a fifth embodiment of the invention.
FIG. 9 is a sectional view of the disk clamp of a sixth embodiment of the invention.
FIG. 10 is a sectional view showing the structure of the conventional disk clamp which holds three recording disks using glass substrates.
FIG. 11 illustrates that the tightening force by the screws for securing the recording disks shown in FIG. 10 is transferred through the disk clamp up to the perimetric portion.
FIG. 12(a) shows a record of fluctuations in data position in a single full recording track after a temperature cycling test is carried out with a disk clamp having six screws.
FIG. 12(b) shows the distribution of data positions in a single full recording track shown in FIG. 12(a) within the width of track.